A NOVEL RECORD OF MICROBIAL CARBON CYCLING IN CARBONATE DUAL CLUMPED ISOTOPES

Biology’s ability to fix inorganic carbon to generate biomass and oxygen is central to the habitability of our planet. Most of our record of past microbial carbon fixation comes from organic matter or fossils, which are not always preserved. Here, we present a new signature of microbial carbon fixation, or autotrophy, recorded in the clumped isotopic compositions (∆₄₇ and ∆₄₈) of carbonate minerals. We found that carbonate formed in the presence of microbes that perform different autotrophic metabolisms record lower ∆₄₇ and higher ∆₄₈ values than theoretical equilibrium values. Models of dual clumped isotope and oxygen isotope kinetics in the DIC system support our finding in modern lake sediments. We found that these same +∆₄₇/-∆₄₈ departures from thermodynamic equilibrium are preserved in Pleistocene to Eocene lacustrine carbonates that have previously been interpreted to have formed in association with photosynthetic organisms. Our interpretation of these signals is that the consumption of CO₂ by autotrophs induces the rapid formation of carbonate minerals that are out of equilibrium with the local inorganic carbon pool. We propose that carbonate dual clumped isotopes could be a powerful tool to identify autotrophic metabolisms in the carbonate rock record, evaluate primary productivity’s relative influence on the Phanerozoic global carbon cycle, and fingerprint chemo- and lithoautotrophy in samples returned from carbonate-bearing Jezero Crater, Mars.